Leptin is synthesized and released from adipose cells in response to adipose tissue changes. It reduces intracellular lipid levels in many types of body cells and thus improves insulin sensitivity. Its principal actions are inhibition of appetite and stimulation of metabolism.
Leptin signals the hypothalamus when there are increases in fat stores. The hypothalamus then restores metabolic balance by decreasing appetite, stimulating physical activity, and burning excess calories. During fasting, leptin levels decrease rapidly and hypothalamus signaling results in an increase in cortisol and a decrease in thyroid, sex, and growth hormones. These actions work together to restore energy balance.
Leptin is usually increased in obesity and leptin resistance may develop. In obesity, appetite suppression does not take place and metabolic rates are lowered. When resistance is present, the leptin secreted in the obese patient is unable to promote energy balance and healthy caloric intake.
